By the Numbers

Multimedia

Related

It’s a fundamental fact of animal and human life, but researchers still do not know for sure why sleep is necessary. Theories abound: Perhaps the body repairs damaged, worn tissues when asleep. Maybe it recharges itself, as if it were a biological battery. Or perhaps sleep affords the brain the opportunity to integrate important facts, memories and emotional impressions recorded from the previous day.

The answer is probably all of the above, but the researchers increasingly are focused on the role sleep plays in stabilizing and consolidating memories. Recent studies appear to catch the process of memory integration in action, and they hint at a neural nightlife that is richer than previously known. The sleeping brain not only sorts important facts from trivia, the findings suggest, but it also replays social interactions and carefully shades experiences with emotional color so they will be more comprehensible.

That’s why interruptions to normal sleep can be so insidious. More than 50 million Americans suffer some sleep problem, from mundane nagging insomnia to more exotic disorders, like bruxism, the official name for teeth grinding, restless leg syndrome or narcolepsy. Of these, a disorder called obstructive sleep apnea causes perhaps the most misery: during the night, the upper airway narrows so much that the body jerks awake, continually gasping for breath.

Each interruption of sleep breaks the spell of “nature’s soft nurse,” in Shakespeare’s phrase. No one who has slept poorly for a week or more is surprised to hear that sleeping problems are linked to physical problems, like high blood pressure, and emotional trouble, especially depression. And those who go long enough without sleep are at risk of a psychotic break, complete with hallucinations.

As neurologists now see it, each awakening throws light into the brain’s darkroom just as an intricate mental photo is being developed. Some of the latest research tries to link stages of sleep to specific functions, like stabilizing memory, so that doctors can better treat sleep-related problems.

Sleep comes in several forms, including deep, or slow-wave, sleep and the shallow, dream-rich state called rapid eye movement, or R.E.M. It is during slow-wave sleep that the body circulates high levels of hormones thought to be involved in tissue repair. And it is during the same stage that the brain seems to reinforce intellectual memories most effectively, like memorized words and numbers.

Recently scientists have gained unprecedented glimpses into the workings of the brain during slow-wave sleep. In one study, German researchers demonstrated that the whiff of a familiar scent helped sleeping students better remember things they had learned the night before. The smell of roses — delivered to people’s nostrils as they studied and, later, as they slept — seemed to intensify the process of normal memory consolidation. When the student were exposed to the fragrance while in slow-wave sleep, an area of their brains called the hippocampus flared with activity, apparently working to make sense of the previous day’s events.

Bursts of the fragrance delivered during R.E.M. sleep had no effect, the researchers found. This makes some sense; the students had studied the location of card pairs, a strictly intellectual task devoid of the narrative or emotional content that so often fills R.E.M.

The dreamy riot of images during R.E.M., some scientists argue, may reflect the brain’s efforts to integrate emotional reactions, in effect attaching mood lighting to a scene whose details might be consolidated in other brain areas.

Researchers in Massachusetts recently analyzed the dream journals of students, comparing entries before and after Sept. 11, 2001. The students’ dreams changed markedly after the attacks, containing four times as many threatening images, many clearly echoing the catastrophe.

“When things are emotional, and context and meaning become important, and you’re thinking about how all these pieces fit together — that seems to migrate to R.E.M.,” said Dr. Robert Stickgold, a psychiatrist at Harvard University.

And a 2005 study, which analyzed dream journals as well as readings from eyelid movement sensors, found that aggressive social interactions were more likely to be processed during R.E.M. than during non-R.E.M. sleep.

Scientists are coming to believe that each stage of sleep is specialized for the brain’s deconstruction and processing of a particular kind of information, and so the fine distinctions between R.E.M. sleep, slow-wave sleep and other stages are increasingly important. While sleep medications now on the market effectively induce sleep, they do not usually increase the slow-wave variety. Pill-induced slumber is far better than insomnia, but it may not truly substitute for naturally deep sleep and the concentrated memory consolidation that it brings.

Individuals vary widely in the amount and quality of sleep they get, and as we age, our bodies seem less able to plumb sleep’s rich depths. Some of us probably are in desperate need of slow-wave sleep; others are likely to be starved for R.E.M.

In the coming years the challenge for sleep research, and its promise, is to give doctors and red-eyed patients opportunities not only to sleep more, but to reshape the quality of the sleep they get.